(1) Field of the invention:
The present invention relates to an oxygen sensor element with a ceramic heater and a method for manufacturing it.
(2) Prior art:
There have been conventionally known oxygen sensors employing an oxygen ion permeable solid electrolyte as a detector of the oxygen concentration in the exhaust gas from the internal combustion engine. In such a type of oxygen sensor, for instance, a cylindrical oxygen sensor element body with one end closed is made of zirconia or the like; electrodes are formed on the inner and outer surfaces of the element; and an electrode-protecting layer is formed at least on the outer electrode while a gas to be measured is allowed to be in contact with the outer electrode, a reference gas is introduced into the interior of the element so as to be in contact with the inner electrode of the element. Thus, the electromotive force is induced due to the difference in oxygen concentration between the reference gas and the gas to be measured, thereby enabling the oxygen concentration of the gas to be measured though the measurement of the electromotive force. A signal representative of the oxygen concentration of the gas measured by the oxygen sensor is transmitted to the fuel supply system, so that feedback control of the air-fuel ratio in the vicinity of the theoretical value is provided.
However, there is a social demand for low fuel consumption in the automobiles. For this purpose, countermeasures for reducing the wearing-out and weight of the automobiles have been taken. However, such countermeasures inevitably lower the exhaust gas temperature. In cases where the exhaust gas temperature is lower, the performance of the oxygen sensor is deteriorated because the oxygen sensor element is not fully heated. Further, it is difficult to control the air-fuel ratio near the theoretical value. Thus, the purification of the exhaust gas is not carried out effectively.
For the purpose of solving the above problems, there have been conventionally proposed oxygen sensors with the heater element (Japanese Patent Application Laid Open Nos. 44999/1980, 134497/1979 and 47753/1981). Such oxygen sensors are broadly classified into two types: one in which the coil-like metal heater is inserted into the inside of the sensor element; and the other in which a bar-like ceramic heater is inserted into the inside of the sensor element.
In the case of the oxygen sensor in which the metal heater is inserted into the inside of the oxygen sensor element, the metal heater of a metal other than Pt is oxidized and therefore, has low life of span. Further, the concentration of the partial pressure of the oxygen in the inside of the sensor element fluctuates due to the oxidation reaction of the metal, and an accurate signal can not be obtained.
There has been also proposed an oxygen sensor in which the metal heater is disposed inside of the thin metal cylindrical body and MgO or the like is filled into the space between the cylindrical body and the heater. Such a heater can prevent the heater itself from being oxidized and consequently the life span of the heater is prolonged. On the other hand, however, the fluctuation of the partial pressure of the oxygen inside of the element can not be avoided due to the oxidation of the metal cylindrical body.
Moreover, in an case of the oxygen sensor employing the ceramic heater in which the heating member is provided on the both sides of the plate-like base of ceramic, since the plate-like ceramic heater base is inserted into the inside of the sensor element, the thickness and width of the base is restricted. Thereby, a satisfactory heating capacity can not necessarily be obtained. If the heater base is designed larger to produce an adequate amount of heat, it is necessary to make larger the inner diameter of the inside of the sensor element to secure the housing space for such a larger heater. In this case the sensor element become more bulky, the oxygen sensor is accordingly larger, with the result of the high cost and enlargement of the space for mounting the oxygen sensor. Besides, the distance between the surface of the plate-like ceramic heater housed inside of the sensor element and the inner surface of the element becomes larger. As a result, the sensor element can not be effectively heated.